A common operation in the foodstuff industries is to heat a particulate foodstuff material, e.g. vegetables, contained in a liquid which is commonly water. For instance, vegetables such as peas or carrot slices are heated to about the boiling point of water for a period to cook the vegetables or alternatively to sterilise them. Because of rubbing contact between the foodstuff particles and the container through which they are transported during heating, the liquid within which the foodstuff particles are contained tends to become discoloured with material abraded from the foodstuff particles. Also, in a continuous system, it is difficult to ensure that the time of passage of the foodstuff particles is sufficiently uniform that none are overcooked if none are to be undercooked.
Apparatus for this purpose is known in GB-A-1223792, in which the particulate product is contained within a horizontal perforate sleeve containing an auger which drives the product along the sleeve. The sleeve is contained in an elongate treatment chamber and a heating liquid is passed into the sleeve along its bottom and withdrawn with the product at its downstream end.
No very intimate contact and mixing between the heated liquid and the product is easily achieved as there is a direct path between the liquid inlet and outlet involving little contact with the particulate material. The liquid and particulate flows are co-current so that the incoming liquid will have to be heated much in excess of the desired end temperature of the product.
Counter current arrangements are shown in GB-A-1453972 and U.S. Pat. No. 4,567,941 in which the sleeve is non-perforate and the liquid is introduced at the downstream (with respect to product conveyance) end of the sleeve and withdrawn at the opposite end.
None of these specifications address the problem of preventing or reducing rubbing of the products. A separate problem left unresolved by these teachings is the need to hold the product at an ideal heat treatment temperature for a desired period once its temperature has been raised and in some cases to reduce the temperature sharply at the end of the heat treatment. It would be desirable to develop forms of apparatus in which, after heating, the product can be kept at a holding temperature and/or cooled in the same treatment chamber.